Impact of Connate Brine Chemistry on In Situ Wettability and Oil Recovery: Pore-Scale Experimental Investigation

In this study, we present the results of micro- and macroscale core-flooding experiments performed on reservoir and outcrop sandstone core samples to investigate the impact of connate brine salinity on wettability and oil recovery. Numerous core samples were fully saturated with connate brines of varying salinities and then subjected to a dynamic wettability restoration (aging) process. The miniature core samples were imaged at high resolutions, and the images were used to measure in situ contact angles and evaluate the wettability alteration. Subsequently, some of the core samples were flooded with identical injection brine to examine the interrelationship between connate brine salinity and oil recovery potentials. The experimental observations demonstrate an ascending trend of initial oil-water contact angles with increases in brine salinity. We believe that brine film thickness, controlled by the DLVO interactions and initial cation-water bridging, is responsible for this wettability sensitivity. The equilibrium wettability state is found to shift toward reduced water-wetness when the connate brine salinity increases, which could be associated with improved oil-mineral bonding at higher salinities. Furthermore, using an ultralow salinity connate brine in the aging process results in a heterogeneous equilibrium wettability state including 46% of weakly water- and neutral-wet pores. It is believed that the ultralow salinity brine preserves stable brine films on some of the pore walls, prevents direct oil-mineral contact, and consequently reduces the degree of wettability alteration during the aging process. The subsequent waterflooding experiments on the aged samples indicated favorable oil recovery from media with heterogeneous equilibrium wettability compared to the weakly oil-wet samples. This was due to the existence of water- and neutral-wet pores that reduced the entry pressures of water-displacing-oil events and enhanced the accessibility of water to the oil-wet pore elements, which in turn improved the oil displacement efficiency.